jcasgen Maven plugin: add option to exclude generating for imported types

Add an option, perhaps something like <skipGenerationForImportedTypes>, which if true (not the default) would only generate those types which were defined directly (that is, not via an import) in the set of type system descriptors specified as input for this plugin.

Rationale from UIMA-1176

Our project reuses a common type system that we got from a different source. The common type system descriptor is imported into our main type system descriptor. The common type system has its own JCas types, in a jar file.
When we generate JCas types for our main type system descriptor, it currently generates all of the classes for all of the imported type systems as well. We don't want this behavior, so we have to manually go through and delete those classes.
I think JCasGen should only generate types for the type system descriptor that you run it on, not on imported type system descriptors.

One way to solve it as described in UIMA-2471

Jg.main0 takes an array of string arguments. To make JCasGen limit the cover classes it generates to just those whose type definitions are contained in some directory (at any sub directory level), you pass 2 additional arguments in this array:
1) "-limitToDirectory" and then following that
2) the path to some directory. Currently this is typically set to a containing Eclipse project directory, for example, when JCasGen is called from the Component Descriptor Editor